The present invention relates to devices for sampling optical signals that are modulated at high frequencies to provide an electrical signal having an amplitude related to the amplitude of the optical signal at the time the sample was taken.
Modulated optical signals are utilized in a variety of communication applications. These signals typically consist of an optical carrier at a fixed carrier wavelength that is modulated to transmit data in the 1 to 10 Gb/sec range; however, data rates of over 300 Gb/sec have been reported. In the following discussion, the frequency at which the carrier is modulated will be referred to as the xe2x80x9cmodulation frequencyxe2x80x9d. In systems that are bandwidth limited, the modulation frequency is the highest frequency component of the modulated signal. The modulation wavelength will be defined as nc/f, where c is the speed of light, n is the index of refraction of the relevant medium and f is the modulation frequency.
To diagnose communication links that utilize such optical signals, a device that converts the optical signal to an electrical signal that can be displayed on a conventional measurement device such as an oscilloscope is required. The simplest solution to this problem would be to convert the optical signal to an electrical signal by applying the light signal to a photodiode. However, conventional electrical measurement devices such as oscilloscopes cannot display signals that vary at the high modulation frequencies described above. Hence, devices that utilize sampling techniques to effectively shift the modulated signal to a lower frequency are utilized. Such devices sample the signal over a sampling time that is small compared to 1/f.
Prior art sampling devices typically utilize a photodetector that is connected in series with a photoconductive switch that is irradiated with a train of short light pulses. Each light pulse corresponds to one sample. Hence, the photoconductive switch samples the output of the photodetector to provide an electrical measurement signal that can be viewed on a conventional measurement device. In prior art devices, each of the photoconductive switch and photodetector is implemented as a photoconducting gap in a strip transmission line. The gaps are illuminated with the two light signals, and hence, the gaps must be separated by a distance that provides sufficient optical isolation to prevent the signal that actuates the photoconductive switch from reaching the gap corresponding to the photodetector and vice versa. The transmission lines are limited to about 1 mm. As the modulation frequency of the optical signal increases, the waveguide alters the potential measured at the photodetector. When the modulation wavelength approaches the distance between the photodetector and the photoconductive switch, a standing wave develops in the waveguide and hence, the potential at the photoconductive switch will differ from that at the photodetector. This difference in potential introduces errors in the sampled signal. As a result, prior art devices based on switch gaps in transmission lines have been limited to modulation frequencies below 20 GHz.
Broadly, it is the object of the present invention to provide an improved measurement device for sampling optical signals.
This and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an apparatus for converting an input optical signal characterized by a modulation frequency and a modulation wavelength to an electrical signal. The apparatus includes a photoconductive switch that is coupled to a photodetector by a common electrode. The photoconductive switch includes a switch optical input, a first switch electrode, and a second switch electrode, the switch connecting the first switch electrode to the second switch electrode in response to a switch light signal received at the switch optical input. The photodetector includes a photodetector optical input for receiving the optical signal, a first photodetector electrode, and a second photodetector electrode, the photodetector causing a current to flow between the first and second photodetector inputs having a magnitude that depends on the intensity of the input optical signal. The connecting electrode connects the first switch electrode to the second photodetector electrode. The photoconductive switch and the photodetector are arranged such that the switch light signal does not interfere with the optical signal at locations proximate to the electrode and the electrode has a length less than 0.5 mm. The connecting electrode is preferably held at a fixed potential relative to the first photodetector electrode. In one embodiment, the connecting electrode is terminated to prevent reflections from occurring at the connecting electrode. The photodetector and the photoconductive switch are preferably integrated on a common substrate.